Autonomous self-powered system for removing thermal energy from pools of liquid heated by radioactive materials, and method of the same

ABSTRACT

An autonomous self-powered system for cooling radioactive materials comprising: a pool of liquid; a closed-loop fluid circuit comprising a working fluid having a boiling temperature that is less than a boiling temperature of the liquid of the pool, the closed-loop fluid circuit comprising, in operable fluid coupling, an evaporative heat exchanger at least partially immersed in the liquid of the pool, a turbogenerator, and a condenser; one or more forced flow units operably coupled to the closed-loop fluid circuit to induce flow of the working fluid through the closed-loop fluid circuit; and the closed-loop fluid circuit converting thermal energy extracted from the liquid of the pool into electrical energy in accordance with the Rankine Cycle, the electrical energy powering the one or more forced flow units.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/450,150, filed Apr. 18, 2012, which claims the benefit of U.S.Provisional Patent Application No. 61/476,624, filed Apr. 18, 2011, theentireties of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to systems and methods ofremoving thermal energy from pools of liquid, and specifically tosystems and methods of removing thermal energy from spent nuclear fuelpools that are self-powered and autonomous.

BACKGROUND OF THE INVENTION

The spent fuel pool (SFP) in a nuclear power plant serves to store usedspent nuclear fuel discharged from the reactor in a deep pool(approximately 40 feet deep) of water. In existing systems, the decayheat produced by the spent nuclear fuel is removed from the SFP bycirculating the pool water through a heat exchanger (referred to as theFuel Pool Cooler) using a hydraulic pump. In the Fuel Pool Cooler, thepool water rejects heat to a cooling medium which is circulated usinganother set of pumps. Subsequent to it's cooling in the Fuel Poolcooler, the pool water is also purified by passing it through a bed ofdemineralizers before returning it to the pool.

In existing systems, the satisfactory performance of the spent fuelcooling and clean up system described above is critically dependent onpumps which require electric energy to operate. As the events at theFukushima Dai-ichi showed, even a redundant source of power such asDiesel generators cannot preclude the paralysis of the classical fuelpool cooling system.

In order to insure that the decay heat produced by the fuel stored inthe SFP is unconditionally rejected to the environment, the presentinvention introduces a heat removal system and method that does notrequire an external source of electric energy or equipment that can berendered ineffective by an extreme environmental phenomenon such as atsunami, hurricane, earthquake and the like.

BRIEF SUMMARY OF THE INVENTION

These, and other drawbacks, are remedied by the present invention. Anautonomous and self-powered system of cooling a pool of liquid in whichradioactive materials are immersed is presented. The inventive systemutilizes a closed-loop fluid circuit through which a low boiling pointworking fluid flows. The closed-loop fluid circuit of the inventivesystem, in accordance with the Rankine Cycle: (1) extracts thermalenergy from the liquid of the pool into the working fluid; (2) convertsa first portion of the extracted thermal energy into electrical energythat is used to power one or more forced flow units that induce flow ofthe working fluid through the closed-loop fluid circuit; and (3)transfers a second portion of the extracted thermal energy to asecondary fluid, such as air. In this way, the inventive system operateswithout the need for any electrical energy other than that which isgenerates internally in accordance with the Rankine Cycle.

In one embodiment, the invention can be an autonomous self-poweredsystem for cooling radioactive materials, the system comprising: a poolat least partially filled with a liquid and radioactive materialsimmersed in the liquid; a closed-loop fluid circuit comprising a workingfluid having a boiling temperature that is less than a boilingtemperature of the liquid, the closed-loop fluid circuit comprising, inoperable fluid coupling, an evaporative heat exchanger at leastpartially immersed in the liquid, a turbogenerator, and a condenser; oneor more forced flow units operably coupled to the closed-loop fluidcircuit to induce flow of the working fluid through the closed-loopfluid circuit; and the closed-loop fluid circuit converting thermalenergy extracted from the liquid of the pool into electrical energy thatpowers the one or more forced flow units; wherein the evaporative heatexchanger comprises: a top header, a bottom header, a downcomer tubedefining a first passageway between the top and bottom headers, and aplurality of heat exchange tubes each forming a second passagewaybetween the top and bottom headers; a working fluid inlet extending intothe downcomer tube for introducing a liquid phase of the working fluidinto the first passageway; and a working fluid outlet for allowing avapor phase of the working fluid to exit the evaporative heat exchanger.

In another embodiment, the invention can be a vertical evaporative heatexchanger for immersion in a heated fluid comprising: a tubeside fluidcircuit comprising: a top header; a bottom header; a core tube forming adowncomer passageway between the top header and the bottom header, thecore tube having a first effective coefficient of thermal conductivity;a plurality of heat exchange tubes forming passageways between thebottom header and the top header, the plurality of the heat exchangetubes having a second effective coefficient of thermal conductivity thatis greater than the first effective coefficient of thermal conductivity;a working fluid in the tubeside fluid circuit; an inlet for introducinga liquid phase of the working fluid into the tubeside fluid circuit; anoutlet for allowing a vapor phase of the working fluid to exit the topheader; and wherein transfer of heat from the heated fluid to theworking fluid induces a thermosiphon flow of the liquid phase of theworking fluid within the tubeside fluid circuit.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic of an autonomous self-powered cooling systemaccording to one embodiment of the present invention;

FIG. 2 is a schematic of an evaporative heat exchanger for use in theautonomous self-powered cooling system of FIG. 1;

FIG. 3 is an induced air-flow air cooled condenser for use in theautonomous self-powered cooling system of FIG. 1;

FIG. 4 is a natural draft air cooled condenser for use in the autonomousself-powered cooling system of FIG. 1;

FIG. 5A is perspective view of the heat exchange tube bundle of thenatural draft air cooled condenser of FIG. 4;

FIG. 5B is a close-up view of area V-V of FIG. 5A; and

FIG. 6 is a transverse cross-section of finned heat exchange tube foruse in the evaporative heat exchanger of FIG. 2 and/or the air cooledcondensers of FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. While the invention is exemplified in FIGS.1-6 as being used to cool pools of liquid in which radioactive materialsare immersed (such as spent nuclear fuel, high level radioactive wasteor low level radioactive waste), the invention is not so limited and canbe used to cool any body of liquid in need of cooling.

Referring first to FIG. 1, an autonomous self-powered cooling system1000 according to an embodiment of the present invention isschematically illustrated. The autonomous self-powered cooling system1000 generally comprises a closed-loop fluid circuit 100, an electricalcircuit 200, and a pool of liquid 50. Radioactive materials 20 areimmersed in the pool of liquid 50, which in the exemplified embodimentis a spent fuel pool. Radioactive materials 20, such as spent nuclearfuel, generate a substantial amount of heat for a considerable amount oftime after completion of a useful cycle in a nuclear reactor. Thus, theradioactive materials 20 are immersed in the pool of liquid 50 to coolthe radioactive materials 20 to temperatures suitable for dry storage.In embodiments where the radioactive materials 20 are spent nuclear fuelrods, said spent nuclear fuel rods will be supported in the pool ofliquid 50 in fuel racks located at the bottom of the pool of liquid 50and resting on the floor. Examples of suitable fuel racks are disclosedin United States Patent Application Publication No. 2008/0260088,entitled Apparatus and Method for Supporting Fuel Assemblies in anUnderwater Environment Having Lateral Access Loading, published on Oct.23, 2008, and United States Patent Application Publication No.2009/0175404, entitled Apparatus or Supporting Radioactive FuelAssemblies and Methods of Manufcturing the Same, published on Jul. 9,2009, the entireties of which are hereby incorporated by reference.

As a result of being immersed in the pool of liquid 50, thermal energyfrom the radioactive materials 20 is transferred to the pool of liquid50, thereby heating the pool of liquid 50 and cooling the radioactivematerials. However, as the pool of liquid 50 heats up over time, thermalenergy must be removed from the pool of liquid 50 to maintain thetemperature of the pool of liquid 50 within an acceptable range so thatadequate cooling of the radioactive materials 20 can be continued.

As discussed in greater detail below, the closed-loop fluid circuit 100extends through the pool of liquid 50. A working fluid 75 is flowedthrough the closed-loop fluid circuit 100. The closed-loop fluid circuit100 extracts thermal energy from the pool of liquid 50 (into the workingfluid 75) and converts the extracted thermal energy into electricalenergy. The electrical energy generated by said conversion powers theelectrical circuit 200, which in turn powers forced flow units 190, 151(described below) that induce flow of the working fluid 75 (FIG. 2)through the closed-loop circuit 100. The aforementioned extraction andconversion of thermal energy into electrical energy is accomplished bythe closed-loop fluid circuit 100 in accordance with the Rankine Cycle.In certain specific embodiments, and depending on the identity of theliquid 50 to be cooled and the working fluid 75 being used, theclosed-loop fluid circuit 100 can accomplish the extraction andconversion of thermal energy into electrical energy in accordance withthe Organic Rankine Cycle.

In order to cool the pool of liquid 50 prior to the liquid 50 of thepool evaporating/boiling, the working fluid 75 is preferably a lowboiling-point fluid (relative to the liquid 50 of the pool). Morespecifically, the working fluid 75 is selected so that it has a boilingtemperature that is less than the boiling temperature of the liquid 50of the pool. It is appreciated that the temperature at which a liquidboils/evaporates is dependent on pressure and that the liquid 50 of thepool and the working fluid 75 may be subject to different pressures incertain embodiments of the invention. Furthermore, as discussed ingreater detail below, the working fluid 75 is evaporated/boiled in anevaporative heat exchanger 110 that is immersed in the pool of liquid50. In certain such embodiments, the liquid 50 of the pool will be undera first pressure and the working fluid 75 in the evaporative heatexchanger 110 will be under a second pressure that is greater than firstpressure. Thus, in such an embodiment, the working fluid 75 is selectedso that the boiling temperature of the working fluid 75 at the secondpressure is less than the boiling temperature of the liquid 50 of thepool at the first pressure. In one specific embodiment, the firstpressure will be atmospheric pressure and the second pressure will be ina range of 250 psia to 400 psia.

In one embodiment, the liquid 50 of the pool is water. As used herein,the term “water” includes borated water, demineralized water and otherforms of treated water or water with additives. Suitable working fluids75 include, without limitation, refrigerants. Suitable refrigerants mayinclude, without limitation, ammonia, sulfur dioxide,chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons,haloalkanes, and hydrocarbons. One particularly suitable refrigerantthat can be used as the working fluid 75 is tetraflouroethane, commonlyknown as HFC-134a.

The exemplified embodiment of the closed-loop fluid circuit 100generally comprise an evaporative heat exchanger 110, a turbogenerator130, a condenser 150, a working fluid reservoir 170, and a hydraulicpump 190. The aforementioned components 110, 130, 150, 170, 190 of theclosed-loop fluid circuit 100 are operably and fluidly coupled togetherusing appropriate piping, joints and fittings as is well-known in theart to form a fluid-tight closed-loop through which the working fluid 75can flow through in both a liquid phase 75A and a vapor phase 75B. Theworking fluid 75 is in the liquid phase 75A between a working fluidoutlet 153 of the condenser 150 and a working fluid inlet 111 of theevaporative heat exchanger 110. The working fluid 75 is in the vaporphase 75B between a working fluid outlet 112 of the evaporative heatexchanger 110 and a working fluid inlet 152 of the condenser 150. Asdiscussed in greater detail below, the evaporative heat exchanger 110,which is immersed in the liquid 50 of the pool, converts the workingfluid 75 from the liquid phase 75A to the vapor phase 75B bytransferring thermal energy from the liquid 50 of the pool into theworking fluid 75. Conversely, the condenser 150 converts the workingfluid 75 from the vapor phase 75B to the liquid phase 75A bytransferring thermal energy from the working fluid 75 into a secondaryfluid (which can be air that is rejected to the environment in certainembodiments).

In the exemplified embodiment, the autonomous self-powered system 1000further comprises two forced flow units that induce flow of the workingfluid 75 through the closed-loop fluid circuit 100, namely the hydraulicpump 190 (which is considered part of the closed-loop fluid circuit 100)and a blower 151 which, when operated, forces cooling air to flow overheat exchange tubes 154 (as shown in FIG. 6) of the condenser 150. Thehydraulic pump 190 directly induces flow of the working fluid 75 throughthe closed-loop fluid circuit 100 by drawing the liquid-phase 75A of theworking fluid 75 from the working fluid reservoir 170 and forcing theliquid-phase 75A of the working fluid 75 into the evaporative heatexchanger 110. The blower 151 indirectly induces flow of the workingfluid 75 through the closed-loop fluid circuit 100 by increasing airflow over the heat exchange tubes 154 of the condenser 150 (the workingfluid 75 being the tubeside fluid in the condenser 150), therebyincreasing the extraction of thermal energy from the working fluid 75 inthe condenser 150 and promoting increased condensation and athermo-siphon flow effect of the working fluid 75. In certainembodiments of the invention, more or less forced flow units can beincorporated into the autonomous self-powered system 1000 as desired.

For example, in certain embodiments, the blower 151 may be omittedwhile, in certain other embodiments, the hydraulic pump 90 may beomitted. For example, if the condenser 50 were a natural draftair-cooled condenser (see FIGS. 4-5B), the blower 151 may be omitted.Furthermore, in certain embodiments where the condenser 50 is not an aircooled condenser, but is for example a shell and tube heat exchanger, ahydraulic pump that is used to force flow of the secondary fluid throughthe condenser 50 can be a forced flow unit.

Irrespective of the exact number and identity of the forced flow unitsthat are used to induce flow of the of the working fluid 75 through theclosed-loop fluid circuit 100, all of said forced flow units are poweredonly by electrical energy generated through the conversion of thethermal energy that is extracted from the liquid 50 of the pool. Morespecifically, in the exemplified embodiment, both the hydraulic pump 190and the blower 151 are operably and electrically coupled to theelectrical circuit 200, which is powered solely by the electrical energygenerated by the turbogenerator 130 (discussed in greater detail below).Thus, the autonomous self-powered system 1000 can operate to cool theliquid 50 of the pool for an indefinite period of time and completelyindependent of any outside sources of electrical energy, other than thatelectrical energy that is generated through the conversion of thethermal energy extracted from the liquid 50 of the pool. Stated simply,the thermal energy of the liquid 50 of the pool is the sole source ofenergy required to drive the cooling system 1000.

Referring still to FIG. 1, the general operation cycle of the autonomousself-powered system 1000 will be described. The working fluid reservoir170 stores an amount of the liquid phase 75 a of the working fluid 75 tocharge and control the quantity of the working fluid 75 in the thermalcycle at start up. The working fluid reservoir 170 also provides themeans to evacuate the closed-loop fluid circuit 100 of air and to fillthe closed-loop fluid circuit 100 with the required amount of theworking fluid 75. In certain embodiments, the working fluid reservoir170 is needed only at the beginning of the system operation (start up)to insure that the proper quantity of the working fluid 75 is injectedinto the thermal cycle.

The hydraulic pump 190 is located downstream of the working fluidreservoir 170 in the exemplified embodiment. However, in alternateembodiments, the hydraulic pump 190 can be located upstream of theworking fluid reservoir 170. Once started, the hydraulic pump 190 drawsthe liquid phase 75A of the working fluid 75 from the working fluidreservoir 170, thereby drawing the liquid phase 75A of the working fluid75 into the working fluid inlet 191 of the hydraulic pump 190. As thehydraulic pump 190 operates, the liquid phase 75A of the working fluid75 is expelled from the working fluid outlet 192 of the hydraulic pumpunder pressure. The expelled liquid phase 75A of the working fluid 75 isforced into the evaporative heat exchanger 110 via the working fluidinlet 111 of the evaporative heat exchanger 110.

The evaporative heat exchanger 110 is at least partially immersed in theliquid 50 of the pool so that thermal energy from liquid 50 can betransferred to the working fluid 70 while in the evaporative heatexchanger 110. In the exemplified embodiment, the evaporative heatexchanger 110 is full immersed in the liquid 50 of the pool.Furthermore, the evaporative heat exchanger 110 is located at a top ofthe pool of liquid 50, which tends to be hotter than the bottom of thepool of liquid 50 due to temperature differentials in the liquid 50 (hotfluids rise). In one embodiment, the evaporative heat exchanger 110 ismounted to one of the sidewalls 55 of the pool of liquid 50 so that theevaporative heat exchanger 110 does not interfere with loading andunloading operations that take place within the pool of liquid 50 forthe radioactive materials 20.

The details of one embodiment of the evaporative heat exchanger 110,including the operation thereof, will now be described with reference toFIGS. 1 and 2 concurrently. Of course, the invention is not so limited,and the evaporative heat exchanger 110 can take on other structuralembodiments in other embodiments of the invention. The evaporative heatexchanger 110 generally comprises a core tube 113 (which acts as adowncomer tube in the exemplified embodiment), a plurality of heatexchange tubes 114, a working fluid bottom header 115, and a workingfluid top header 116, which collectively define a tubeside fluidcircuit. The working fluid bottom header 115 comprises a bottom tubesheet 117 while the working fluid top header 116 comprises a top tubesheet 118.

In one embodiment, the bottom and top headers 115, 116 and the core pipe113 are constructed of a corrosion resistant alloy, such as stainlesssteel. The bottom and top tube sheets are constructed of an aluminumclad stainless steel. The heat exchange tubes 114 are constructed ofaluminum (as used herein the term “aluminum” includes aluminum alloys)and are welded to the aluminum cladding of the bottom and top tubesheets 117, 118 to make leak tight joints. The core pipe 113 will bewelded to the stainless steel base metal of the bottom and top tubesheets 117, 118. Of course, other materials and constructionmethodologies can be used as would be known to those of skill in theart.

The core tube 113 extends from the working fluid outlet header 116 tothe working fluid inlet header 115, thereby forming a fluid-tight pathbetween the two through which the liquid phase 75A of the working fluid75 will flow. More specifically, the core tube 113 is connected to thelower and upper tube sheets 117, 118 of the working fluid headers 115,116. The working fluid inlet 111 extends into the core tube 113 andintroduces cool liquid phase 75A of the working fluid 75 into a topportion of the core tube 113. The core tube 113 is formed of a materialthat has a low coefficient of thermal conductivity (as compared to thematerial of which the heat exchange tubes 114 are constructed), such assteel. The core tube 113 may also comprise a thermal insulating layer,which can be an insulating shroud tube, to minimize heating of theliquid phase 75A of the working fluid 75 in the core tube 113 by theliquid 50 of the pool. Irrespective of the materials and/or constructionof the core tube 113, the core tube 113 has an effective coefficient ofthermal conductivity (measured from an inner surface that is contactwith the working fluid 75 to an outer surface that is in contact withthe liquid 50 of the pool) that is less than the effective coefficientof thermal conductivity of the heat exchange tubes 114 (measured from aninner surface that is contact with the working fluid 75 to an outersurface that is in contact with the liquid 50 of the pool) in certainembodiments of the invention. As discussed in detail below, this helpsachieve an internal thermosiphon recirculation flow of the liquid phase75A of the working fluid 75 within the evaporative heat exchanger 110itself (indicated by the flow arrows in FIG. 2).

The plurality of heat exchange tubes 114 form a tube bundle thatcircumferentially surrounds the core tube 113. The plurality of heatexchange tubes 114 are arranged in a substantially vertical orientation.The heat exchange tubes 114 are constructed of a material having a highcoefficient of thermal conductivity to effectively transfer thermalenergy from the liquid 50 of the pool to the working fluid 75. Suitablematerials include, without limitation, aluminum, copper, or materials ofsimilar thermal conductivity. In one embodiment, the heat exchange tubes114 are finned tubes comprising a tube portion 119 and a plurality offins 120 extending from an outer surface of the tube portion 119 (shownin FIG. 6). In the exemplified embodiment, each heat exchange tube 114comprises four fins 120 extending from the tube portion 119 at points of90 degree circumferential separation.

During operation of the autonomous self-powered system 1000, cool liquidphase 75A of the working fluid 75 enters the evaporative heat exchanger110 via the working fluid inlet 111 as discussed above. The liquid phase75A of the working fluid 75 is considered “cool” at this time because ithad been previously cooled in the condenser 50. As the cool liquid phase75A of the working fluid 75 enters the evaporative heat exchanger 110,it is introduced into the core tube 113. The cool liquid phase 75A ofthe working fluid 75 flows downward through the core tube and into thebottom header 115, thereby filling the bottom header 115 and flowingupward into the plurality of heat exchange tubes 114. As the liquidphase 75A of the working fluid 75 flows upward in the plurality of heatexchange tubes 114, thermal energy from the liquid 50 of the pool thatsurrounds the plurality of heat exchange tubes 114 is conducted throughthe plurality of heat exchange tubes 114 and into the liquid phase 75Aof the working fluid 75, thereby heating the liquid phase 75A of theworking fluid 75. The warmed liquid phase 75A of the working fluid 75then enters the top header 116 where it is drawn back into the core tube113 by a thermosiphon effect. As a result, the liquid phase 75A of theworking fluid 75 is recirculated back through the aforementioned cycleuntil the liquid phase 75A of the working fluid 75 achieves the boilingtemperature of the working fluid 75, thereby being converted into thevapor phase 75B of the working fluid 75. The vapor phase 75B of theworking fluid 75 rises within the evaporative heat exchanger 110 andgather within a top portion of the top header 116 where it then exitsthe evaporative heat exchanger 110 via the working fluid outlet(s) 112.The internal design of the evaporative heat exchanger 110 promotesrecirculation of the working fluid 117 and separation of the vapor phase75B from the liquid phase 75A in the top header 116 (as shown in FIG.2).

As mentioned above, the evaporative heat exchanger 110 is pressurized toa supra-atmospheric pressure. In one embodiment, the pressure within theevaporative heat exchanger 110 is between 250 psia to 400 psia, with amore preferred range being between 280 psia and 320 psia, withapproximately 300 psia being most preferred. Pressurization of theevaporative heat exchanger 110 is achieved through properly positionedvalves as would be known to those of skill in the art. In oneembodiment, the working fluid 75 and the pressure within the evaporativeheat exchanger 110 are selected so that the working fluid evaporates ata temperature between 145° F. and 175° F., and more preferably between155° F. and 165° F.

Referring solely now to FIG. 1, the pressurized vapor phase 75B of theworking fluid 75 exits the working fluid outlet 112 of the evaporativeheat exchanger 110 and enters the working fluid inlet 131 of theturbogenerator 130. The pressurized vapor phase 75B of the working fluid75 produced in the evaporative heat exchanger 110 then serves toenergize a suitably sized turbogenerator 130. In other words, theturbogenerator 130 converts a first portion of the thermal energyextracted from the liquid 50 of the pool (which is now in the form ofkinetic energy (velocity head) and/or potential energy (pressure head)of the vapor flow) to electrical power, as would be understood by thoseof skill in the art. As used herein, the term “turbogenerator” includesa device and/or subsystem that includes a turbine and electricalgenerator either in directed or indirect connection. The term“turbogenerator” is intended to include any device and/or subsystem thatcan convert the pressurized vapor phase 75B of the working fluid 75 intoelectrical energy. As the vapor phase 75B of the working fluid 75 passesthrough the turbogenerator 130 it is partially depressurized as it exitsthe working fluid outlet 132 of the turbogenertaor still in the vaporphase 75B. At this point, the vapor phase 75B of the working fluid 75may be at a pressure between 200 psia and 270 psia.

As mentioned above, the forced flow units (which in the exemplifiedembodiment are the hydraulic pump 190 and the blower 151) are operablyand electrically coupled to the turbogenerator 130 by the electricalcircuit 130 via electrical lines 201. All of the forced flow units arepowered solely by the electrical energy generated by the turbogenerator130 as discussed above. Moreover, in many instances, the turbogenerator130 will generate surplus electrical energy. Thus, the autonomousself-powered system 1000 may further comprise a rechargeable electricalenergy source 202, such as a battery, operably and electrically coupledto the turbogenerator 130 by the electrical circuit 200. In certainembodiments, the rechargeable electrical energy source 202 will beoperably coupled to a controller so that certain valves, sensors, andother electrical components can be operated even when the turbogenerator130 is not running.

Referring still to FIG. 1, the partially depressurized vapor phase 75Bof the working fluid 75 that exits the turbogenerator 130 enters theworking fluid inlet 152 of the condenser 150. The condenser 150transfers a sufficient amount of thermal energy from the partiallydepressurized vapor phase 75B of the working fluid 75 to a secondaryfluid so that the depressurized vapor phase 75B of the working fluid 75is converted back into the liquid phase 75A of the working fluid 75. Thecondensed liquid phase 75A of the working fluid 75 exits the condenser150 via the working fluid outlet 153 of the condenser where it flowsback into the working fluid reservoir 170 for recirculation through theclosed-loop fluid circuit 100. In one embodiment, the condenser 150 isan air-cooled condenser and, thus, the secondary fluid is air that isexpelled to the environment. In other embodiments, the condenser 150 canbe any type of heat exchanger than can remove thermal energy from thepartially depressurized vapor phase 75B of the working fluid 75,including without limitation, a shell and tube heat exchanger, a plateheat exchanger, a plate and shell heat exchanger, an adiabatic heatexchanger, a plate fin heat exchanger, and a pillow plate heatexchanger.

Referring to FIGS. 1 and 3 concurrently, an example of induced flow aircooled-condenser 150 that can be used in the system 1000 is exemplified.The induced flow air cooled-condenser 150 comprises a plurality of heatexchange tubes 154 (FIG. 6) positioned within an internal cavity formedby a housing 159. The working fluid 75 is the tubeside fluid and flowsthrough the plurality of heat exchange tubes 154. The plurality of heatexchange tubes 154 are arranged in a substantially vertical orientationand are finned as discussed above with respect to the heat exchangetubes 114 of the evaporative heat exchanger 110, and as shown in FIG. 6.

The induced flow air cooled-condenser 150 comprises a cool air inlet 155and a warmed air outlet 156. The warmed air outlet 156 is at a higherelevation than the cool air inlet 155. The plurality of heat exchangetubes 154 are located in the cavity of the housing at an elevationbetween the elevation of the cool air inlet 155 and an elevation of thewarmed air outlet 156. As such, in addition to the air flow within thehousing 159 being forced by operation of the blower 151, which islocated within the warmed air outlet 156, additional air flow will beachieved by the natural convective flow of the air as it is heated(i.e., the chimney effect). As warmed air exists the condenser 150 viathe warmed air outlet 156, additional cool air is drawn into the coolair inlet 155. The induced flow air cooled-condenser 150, in certainembodiments, is located outside of the containment building in which thepool of liquid 50 is located.

Referring now to FIGS. 4-5B concurrently, an example of natural draftair cooled-condenser 250 that can be used in the system 1000 isexemplified. Of note, the flow of air over the heat exchanger tubes 154(which are also vertically oriented) is accomplished solely by naturalconvection (i.e., the chimney effect) and, thus, the blower 151 is notrequired. However, in certain embodiments, the blower 151 can beincorporated into the natural draft air cooled-condenser 250 as desiredto accommodate for situations where the ambient air may reach elevatedtemperatures that could negatively affect adequate heat removal from theworking fluid 75. Of further note, the natural draft aircooled-condenser 250 comprises a working fluid inlet header 260comprising a plurality concentrically arranged toroidal tubes.Similarly, the natural draft air cooled-condenser 250 also comprises aworking fluid outlet header 261 comprising a plurality concentricallyarranged toroidal tubes. The plurality of heat exchange tubes 154 form atube bundle that extends from the toroidal tubes of the working fluidinlet header 260 to the toroidal tubes of the working fluid outletheader 261.

As with the air-cooled condenser 150, the natural draft aircooled-condenser 250 comprises a cool air inlet 255 and a warmed airoutlet 256. The warmed air outlet 256 is at a higher elevation than thecool air inlet 255. The plurality of heat exchange tubes 254 are locatedin the cavity of the housing 259 at an elevation between the elevationof the cool air inlet 255 and an elevation of the warmed air outlet 256.

The system 1000 of the present invention can be used to remove heat fromany pool of water. In particular, it can be used to reject the decayheat from a spent fuel pool. Because the inventive system 1000 does notrequire any external active components such as pumps, motors, orelectric actuators/controllers, it can be engineered as an autonomoussystem that is not reliant on an external energy source to function.Thus, the inventive system 1000 is safe from an extreme environmentalevent such as a tsunami. It is evident that several of the systems 1000can be deployed in a single pool of liquid if desired.

The inventive system 1000 can be retrofit to existing plants for useboth as an emergency cooling system under station blackout scenarios andas an auxiliary system to provide operational flexibility duringcorrective and elective maintenance (particularly during outages). Theinventive system 1000 can also be incorporated into the plant design fornew build projects to operate as the primary cooling system, therebyremoving station blackout as a possible threat to spent fuel poolsafety.

As used throughout, ranges are used as shorthand for describing each andevery value that is within the range. Any value within the range can beselected as the terminus of the range. In addition, all references citedherein are hereby incorporated by referenced in their entireties. In theevent of a conflict in a definition in the present disclosure and thatof a cited reference, the present disclosure controls.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques. It is tobe understood that other embodiments may be utilized and structural andfunctional modifications may be made without departing from the scope ofthe present invention. Thus, the spirit and scope of the inventionshould be construed broadly as set forth in the appended claims.

What is claimed is:
 1. An autonomous self-powered system for coolingradioactive materials, the system comprising: a pool at least partiallyfilled with a liquid and radioactive materials immersed in the liquid; aclosed-loop fluid circuit comprising a working fluid having a boilingtemperature that is less than a boiling temperature of the liquid, theclosed-loop fluid circuit comprising, in operable fluid coupling, anevaporative heat exchanger at least partially immersed in the liquid, aturbogenerator, and a condenser; one or more forced flow units operablycoupled to the closed-loop fluid circuit to induce flow of the workingfluid through the closed-loop fluid circuit; and the closed-loop fluidcircuit converting thermal energy extracted from the liquid of the poolinto electrical energy that powers the one or more forced flow units;wherein the evaporative heat exchanger comprises: a top header, a bottomheader, a downcomer tube defining a first passageway between the top andbottom headers, and a plurality of heat exchange tubes each forming asecond passageway between the top and bottom headers; a working fluidinlet extending into the downcomer tube for introducing a liquid phaseof the working fluid into the first passageway; and a working fluidoutlet for allowing a vapor phase of the working fluid to exit theevaporative heat exchanger.
 2. The autonomous self-powered system ofclaim 1 wherein the downcomer tube and the heat exchange tubes of theevaporative heat exchanger are arranged in a substantially verticalorientation.
 3. The autonomous self-powered system of claim 2 whereinthe downcomer tube is circumferentially surrounded by the heat exchangetubes.
 4. The autonomous self-powered system of claim 1 wherein thedowncomer tube comprises a thermal insulating layer.
 5. The autonomousself-powered system of claim 1 wherein the liquid in the pool is indirect contact with an outer surface of at least one of the plurality ofheat exchange tubes.
 6. The autonomous self-powered system of claim 1wherein the first passageway has a first width and the each of thesecond passageways have a second width, the first width being greaterthan the second width.
 7. The autonomous self-powered system of claim 1wherein the downcomer tube has a first effective coefficient of thermalconductivity and the heat exchange tubes have a second effectivecoefficient of thermal conductivity, the first effective coefficient ofthermal conductivity being less than the second effective coefficient ofthermal conductivity.
 8. The autonomous self-powered system of claim 1wherein the first passageway is directly fluidly coupled to the topheader, the bottom header, and the working fluid inlet, and wherein eachof the second passageways are directly fluidly coupled to the top andbottom headers.
 9. The autonomous self-powered system of claim 1 whereinthe working fluid inlet extends through the top header and into thedowncomer tube to isolate the working fluid being introduced into theevaporative heat exchanger via the working fluid inlet from the workingfluid already circulating within the evaporative heat exchanger.
 10. Theautonomous self-powered system of claim 1 wherein the evaporative heatexchanger is configured to achieve an internal thermosiphon flow of theliquid phase of the working fluid within the evaporative heat exchanger.11. The autonomous self-powered system of claim 1 further comprising:the evaporative heat exchanger converting the working fluid from aliquid phase to a vapor phase by transferring the thermal energy fromthe liquid in the pool to the working fluid; the turbogeneratorreceiving the vapor phase of the working fluid from the evaporative heatexchanger, the turbogenerator generating the electrical energy byextracting energy from the vapor phase of the working fluid flowingthrough the turbogenerator; the condenser receiving the vapor phase ofthe working fluid from the turbogenerator and converting the vapor phaseof the working fluid flowing through the condenser back into the liquidphase of the working fluid by removing thermal energy from the workingfluid; and the at least one forced flow unit electrically coupled to theturbogenerator so as to be powered by the electrical energy generated bythe turbogenerator.
 12. The autonomous self-powered system of claim 1wherein the one or more forced flow units comprises a hydraulic pump,the closed-loop fluid circuit comprising the hydraulic pump, thehydraulic pump forcing the liquid phase of the working fluid into theevaporative heat exchanger.
 13. The autonomous self-powered system ofclaim 1 wherein the liquid in the pool is at a first pressure and theworking fluid within the evaporative heat exchanger is at a secondpressure that is greater than the first pressure, the boilingtemperature of the working fluid at the second pressure being less thanthe boiling temperature of the liquid in the pool at the first pressure.14. The autonomous self-powered system of claim 13 wherein the firstpressure is atmospheric and the second pressure is in a range of 250psia to 400 psia.
 15. The autonomous self-powered system of claim 1wherein the liquid in the pool is water and the working fluid isselected from a group consisting of a refrigerant and a hydrocarbon 16.The autonomous self-powered system of claim 1 wherein the evaporativeheat exchanger is fully immersed in the liquid in the pool and locatedat a top portion of the pool.
 17. The autonomous self-powered system ofclaim 1 further comprising a rechargeable electrical energy sourcecoupled to the turbogenerator so as to be charged by the electricalenergy generated by the turbogenerator.
 18. The autonomous self-poweredsystem of claim 1 wherein the autonomous self-powered system operatesfree of electrical energy generated outside of the closed-loop fluidcircuit.
 19. A vertical evaporative heat exchanger for immersion in aheated fluid comprising: a tubeside fluid circuit comprising: a topheader; a bottom header; a core tube forming a downcomer passagewaybetween the top header and the bottom header, the core tube having afirst effective coefficient of thermal conductivity; a plurality of heatexchange tubes forming passageways between the bottom header and the topheader, the plurality of the heat exchange tubes having a secondeffective coefficient of thermal conductivity that is greater than thefirst effective coefficient of thermal conductivity; a working fluid inthe tubeside fluid circuit; an inlet for introducing a liquid phase ofthe working fluid into the tubeside fluid circuit; an outlet forallowing a vapor phase of the working fluid to exit the top header; andwherein transfer of heat from the heated fluid to the working fluidinduces a thermosiphon flow of the liquid phase of the working fluidwithin the tubeside fluid circuit.
 20. The vertical evaporative heatexchanger of claim 19 wherein the inlet introduces the liquid phase ofthe working fluid directly into the downcomer passageway of the coretube.